The present invention relates to the technical field of sound-attenuating ear plugs and in particular to the kind which comprises an elongate body of elastic material, which is adapted to be inserted into the auditory meatus or canal of a user""s ear. The invention also relates to a method of manufacturing an ear plug. The invention is particularly directed to indicating an ear plug having an improved attenuation characteristic.
Here the term xe2x80x9cplugxe2x80x9d refers to an ear protector which, when used, is at least partially inserted into an auditory meatus of a user""s ear, in contrast to earmuffs which are to be applied to the outside of the user""s ear.
In the technical field of ear plugs, it is known that it is desirable but difficult to obtain an attenuation curve which is substantially frequency-independent or which at least does not vary excessively with varying frequency. FIG. 1 of the accompanying drawings shows, by means of a curve marked xe2x80x9cIdealxe2x80x9d, an example of such an ideal attenuation characteristic which in this example is at a constant attenuation value of 17 dB up to 8 kHz. However, prior-art ear plugs exhibit attenuation characteristics which are rather far from such an ideal-straight characteristic. A particular problem is that the attenuation is often undesirably high within the frequency range that is most important to the perception of speech, i.e. substantially in the range of 1 kHz to 4 kHz (marked with vertical lines in FIG. 1). This problem is discussed, for instance, in SE 8102931-6 (Racal), in which the aim is a substantially straight attenuation characteristic up to 2 kHz. A generally known problem is that it is difficult to provide suitable low-frequency attenuation while not excessively attenuating high-frequency sound.
The Applicant of the present application currently manufactures and sells glass xe2x80x9cdownxe2x80x9d ear plugs of the kind which is described in EP 0 050 601 and which is schematically shown in FIG. 2 of the accompanying drawings. The ear plug consists of an elongate body 3 which is made of elastic material and which is surrounded by a sheath 1 of flexible plastic material. Referring to its direction of insertion into the auditory meatus, the plug has a front end A and a rear end B.
The sheath 1 serves to facilitate the handling of the plug and to provide improved hygiene. The sheath 1 also helps to maintain the desired shape of the elastic body 3. At the rear end B of the plug, the sheath 1 has a circumferential more rigid portion 7, such as an outwardly directed collar or flange portion. The sheath 1 is made of a deep-drawn, thin and flexible film made of, for instance, polyvinyl chloride (PVC), polyurethane (PUR) or polyethylene (PE). Since the sheath is thin and flexible, a satisfactory adjustment is obtained between the elastic body and the inner wall of the auditory meatus.
The elastic body of the ear plug which is currently manufactured is made of glass down and is considerably blocking as regards air-flow, i.e. very airtight and thus provides a relatively high sound-attenuating effect. The curve which is marked xe2x80x9cPrior-art glass down plugxe2x80x9d in the diagram in FIG. 1 shows the result of a measurement of the attenuation (according to ISO 4869-1) of this prior-art glass down plug at the frequencies which are specifically indicated on the x-axis in the diagram. It is to be noted that the attenuation characteristic of the plug deviates considerably from the ideal, straight characteristic, in particular in the range of speech where the attenuation is considerably higher.
The Applicant of the present application presently manufactures and sells also elastic ear plugs made of foam material. The curve marked xe2x80x9cPrior-art foam plugxe2x80x9d in the diagram in FIG. 1 shows the result of a measurement of the attenuation of this prior-art foam plug. Admittedly, the characteristic is somewhat straighter than in the case of the glass down plug of FIG. 2, but the attenuation of the foam plug is not lower in the range of speech but on the contrary higher, and therefore the perception of speech as such is not improved. For higher frequencies, the attenuation amounts to about 40 dB.
It is an object of the invention to provide a sound-attenuating ear plug having improved attenuation characteristic. A particular object of the invention is to indicate an ear plug which reduces the above-identified problems concerning (i) nonlinear or frequency-dependent attenuation and (ii) excessive attenuation in the range of speech.
These objects are achieved by means of an ear plug having the features defined in the appended claims.
According to one aspect of the invention, a sound-attenuating ear plug is provided, comprising an elongate elastic body adapted to be inserted at least partially into an auditory meatus, said body being essentially non-attenuating as regards sound, and a foil held by said body, as seen along a longitudinal axis of said body, in a three-dimensionally rounded concave or convex shape which provides such acoustical properties that the foil is responsible for the major part of the sound-attenuating effect of the ear plug.
According to another aspect of the invention, a sound-attenuating ear plug is provided, comprising an elastic body adapted to be inserted into an auditory meatus, said elastic body being made of a material which is substantially non-attenuating as regards sound, and a sheath which at least partially surrounds the elastic body, at least a part of said sheath having acoustical properties being responsible for the major part of the sound-attenuating effect of the ear plug.
According to a further aspect of the invention, a method is stated of manufacturing an ear plug, which comprises an elongate body made of elastic material and surrounded by a sheath made of flexible plastic material, a thermoplastic plastic film or foil being deep-drawn into a sheath in a manner known per se and the sheath, subsequently or simultaneously, being filled with elastic material. According to this method, as said elastic material, use is mainly made of a material which is substantially non-blocking as regards air-flow.
According to yet another aspect of the invention, use, in an ear plug, of an airtight foil is provided, which foil is three-dimensionally rounded in the direction of the auditory meatus and is the component of the ear plug that is responsible for the major part of the sound-attenuating effect of the ear plug.
According to an additional aspect of the invention, a method is provided of controlling the sound-attenuating effect of a sound-attenuating ear plug, comprising the steps of providing the ear plug with a foil which is three-dimensionally rounded in the longitudinal direction of the auditory meatus, and choosing the thickness of the rounded foil so as to obtain a desired sound-attenuating effect of the ear plug.
The invention is based on the understanding that it is possible to obtain an improved attenuation characteristic by lowering the air-flow blocking properties of an elastic body included in the plug, and instead using a rounded foil to provide the major part of the attenuation effect of the ear plug. This is apparently a solution which goes against traditional practice within the field, where elastic materials having relatively good air-flow blocking properties are normally desired to provide the sound-attenuating effect of the plug. It has thus been found that if, for instance, the starting-point is the prior-art glass down plug which is described above with reference to FIG. 2 and which consists, on the one hand, of a thin plastic sheath and, on the other, of a glass down body having a high flow resistance, and this glass down body is replaced by an elastic body which is substantially non-blocking as regards air-flow, a sound-attenuating ear plug is provided having considerably improved frequency characteristic. The attenuation characteristic of an ear plug according to the invention can be made much flatter (more frequency-independent) compared with the attenuation characteristic of prior-art plugs, and in spite of this the attenuation can be kept on a suitable level. The background of this remarkable effect will be explained in greater detail below.
The invention is also based on the understanding that the attenuation characteristic can be improved if the foil is made to be rounded or dome-shaped in the direction of the auditory meatus. The rounded shape of the foil increases its acoustic stiffness. In particular, the acoustic stiffness can thus be increased without increasing the weight. Therefore, also a very thin and light foil can be used to provide the major part of the sound-attenuating effect of the ear plug. This property of the rounded foil can be compared to the increased strength of a Roman arch due to its arched shape. The positive effects of this acoustic stiffness on the attenuation characteristic will be explained in more detail below. In a preferred embodiment, the foil is rounded towards the eardrum, but the foil can also be rounded in the other direction, i.e. concave towards the eardrum.
During use, when the ear plug is placed in the auditory meatus, there should be substantially no transport of air between the outer ear and the inner space defined by the ear drum, the auditory meatus and the plug. Thus, the foil either independently provides the air-blocking function of the plug, or contributes to said function by e.g. being a portion of a sheath surrounding the elongate elastic body which is essentially non-blocking as regards air. The foil, which is suitably substantially airtight, extends transversely of a longitudinal axis of said body, and preferably across the central longitudinal axis of-said body.
In an advantageous embodiment of the invention a dome-shaped foil is applied on the front tip portion of the elongate elastic body. The foil is suitably, at least partially, freely movable in relation to the underlying body which provides the general shape of the dome-shaped foil.
Acoustic stiffness and acoustic resistance are two forms of acoustic impedance, which will be discussed in the following. As has been previously described, the elongate elastic body is substantially non-blocking as regards air-flow, i.e. the elastic body has a high flow resistance. The flow resistance of the elastic material which expands and distends the foil is suitably described by means of acoustic resistance Ra. In the relevant frequency range, this acoustic resistance Ra is not to be larger than the acoustic stiffness of the rounded foil.
The acoustic stiffness of the foil is frequency dependent and can be best described by an analogue to electrical capacitance. The acoustical analogue to electrical capacitance is acoustic compliance Ca, which is a frequency independent constant. The acoustic stiffness Ka is given by:       K    a    =      1          j2π      ⁢              xe2x80x83            ⁢              fC        a            
Thus, a large acoustic stiffness means a small acoustic compliance Ca. To obtain a sufficient sound-attenuating effect for the rounded-foil, it should have an acoustic compliance Ca which is less than or equal to 1.7*10xe2x88x9211 m5/N.
As explained above, in order for the rounded foil to provide for the major part of the sound-attenuating effect, the acoustic resistance Ra of the material of the elastic body is not to be larger than the acoustic stiffness Ka of the rounded foil. A suitable acoustic resistance Ra in the relevant attenuation range of the ear plug-according to the invention has been found to be less than or equal to 1.2*109 sN/m5.
A particular advantage of the invention is that the thickness of the rounded foil can be used as a manufacturing parameter to control the sound-attenuating effect of the ear plug. In particular, the attenuation characteristic can be substantially parallel-displaced if the thickness of the foil is altered.
Preferred embodiments of the invention are defined in the dependent claims.
A preferred embodiment of an ear plug according to the invention can be manufactured according to the specifications concerning shape, foil material, deep-drawing technique, etc. which are described in above-mentioned EP 0 050 601. The sheath may thus be deep-drawn from a thermoplastic plastic film or foil made of, for instance, PVC, PUR or PE. The plastic material should have high values of breaking stress and elongation at break and a relatively low value of yield stress. If a deep-drawing technique is used in the manufacture, the plastic material should also have a small degree of shrinkage after the deep-drawing. If the ear plug according to the invention has a surrounding sheath, this can be produced with varying thickness distribution, such as described in this document.
Thus, it should be understood from the foregoing that the main function of the elastic body of an ear plug according to the invention, is to stretch or distend the foil and, in a preferred embodiment, to shape it around the tip of the body into a dome-shaped, three-dimensionally rounded foil which xe2x80x9cpointsxe2x80x9d in the direction of a longitudinal axis of the body.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific example, while indicating a preferred embodiment of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Below, a preferred embodiment of an ear plug according to the invention will be described as well as the physical principles on which the invention is based.